1. Field of the Invention
The invention relates to the field of airborne optoelectronic systems, notably placed on board military aircraft for 3D identification and localization and, more particularly, to systems of this type having facetted ports.
2. Description of the Prior Art
Conventionally, an optoelectronic system for 3D identification and/or localization is used in a warplane. This system has a port for shielding the camera constituted by an optical device associated with an optical sensor and means for processing the signal that comes from the sensor. This port may take various forms, according to requirements: it may be spherical, plane or facetted, i.e. formed by the juxtaposition of planes that are inclined with respect to one another.
The spherical shape is generally used for its aerodynamic profile and because it permits a wide range of deflection of the line of sight (LOS). Moreover, this shape has the advantage of high resistance to aerodynamic pressures. However, owing to its refractive power, a spherical port has to be fixedly joined to the optical assembly of the camera. The optical axis of the port should remain aligned with that of the optical device of the optoelectronic system, irrespectively of the mechanical and climatic environment, so as not to reduce the image quality. A spherical port such as this is not appropriate when the mechanical constraints dictate an off-axis line of sight.
A plane port circumvents this drawback. In particular, it enables the camera to be suspended from the port-bearing structure. The need to suspend the camera is related to the mechanical environment. For, this camera is generally associated with a line-of-sight orienting device that is stabilized with respect to the ground-related reference in order to obtain an image of the external scene that is "stable" for the pilot's eyes in the case of airborne systems. The plane port has no refractive power and, consequently, it is neutral and enables a shifting of the orientation device in relation to the port without affecting the image quality. However, the plane port has the drawback of restricting the range of deflection of the line of sight for two main reasons related to the necessary restriction of the size of the plane port, firstly because of aerodynamic considerations and, secondly, because of the Brewster limit angle.
A standard approach used to increase the angles of deflection with a plane port consists in making the port movable about one or two rotational axes, by means of a follower cap:
With a "single-axis" follower cap, the plane port maintains a constant angle with the direction of the line of sight when the latter undergoes a rotation about an axis. The device for orienting the line of sight is generally movable about two orthogonal axes (elevation and relative bearing). The "single-axis" follower cap consequently permits a major angular deflection of the line of sight about a single axis while at the same time enabling the optical field to permanently go through a single plane port. By contrast, the angular deflection of the line of sight along the other axis is more restricted. PA1 an inherent automatic control device (fulfilling the driving, control and copying functions). PA1 a greater space factor with an increase in the coefficient of penetration in air, PA1 conditions of imperviousness, compatible with the rotation, which are generally costly given that the optoelectronic systems require a stable pressurization irrespectively of the external environment. PA1 wide angles of deflection of the line of sight; PA1 the suspension of the assembly formed by the line-of-sight orientation device, the optical device and the sensor. PA1 wide angles of deflection of the line of sight, PA1 suspension of the optical bank, in using a facetted and compensated port to eliminate the dihedron effect while at the same time preserving constant pressure within the optical device and preserving the optical qualities of the system.
With a "dual-axis" follower cap, the plane port is borne, this time, by a structure that is movable in two rotational axes (elevation and relative bearing) enabling the optical field to permanently go through a single plane port, irrespectively of the direction of the line of sight. This permits great angular deflections along both axes (elevation and relative bearing).
The follower cap is a movable port that in no way modifies the principle and the structure of the device for orienting the line of sight of the optoelectronic system. This system remains shielded from aerodynamic effects by the "follower" port. This is very important for the quality of stabilization of the line of sight. However, another approach has the plane port borne by the line-of-sight orientation device. This has the advantage of reducing size of the driving system (the number of motors to be automatically controlled) and the amount of space occupied when the stabilization performance characteristics do not have priority and may be of lower quality.
These devices do not provide for associating simplicity, space factor and performance characteristics, all at the same time. The follower caps require:
The borne caps are generally not suited to airborne systems either, given the performance characteristics, and they require a tight-sealing device as do the preceding ones.
The facetted shape makes it possible to combine the advantages of the two previous shapes (spherical and plane) without having their drawbacks (namely the need to maintain imperviousness in rotation and the need for an automatically controlled driving system), while preserving the advantages of the fixed port, for a system with high angular deflection. It permits:
By contrast, the facetted shape has the drawback, for a port of an airborne optoelectronic system, of doubling the image of the target to be identified at high altitude, in doing so when the field of the camera intercepts an juncture between two facets. Furthermore, when a telemeter is used for the 3D localization, the divergence of the laser beam is also increased under the same conditions: this affects the range performance characteristics of the system.
This doubling phenomenon is called the "dihedron effect" and is due to the difference between the refractive indices of the gases on either side of the port, prompted by the difference in pressure between the interior of the system and the exterior: the interior of the system must be kept at a constant pressure so that it does not modify the characteristics of the optical device while the exterior of the port undergoes the variations in atmospheric pressure with altitude.